The present invention relates to a method of decoloring a colored concrete. More particularly, the present invention relates to a method of decoloring a colored concrete residue which attaches to or remains in production facilities in order to return a color thereof to that of an ordinary plain concrete, without works or treatments for washing-out or removing the colored concrete residue which attaches to or remains in the production facilities, in which the works or treatments are required in the case where the production facilities are used for the production of an ordinary plain concrete after the colored concrete has been produced therein, and without works or treatments for purifying a waste water generated upon washing-out or removing the residue and for making the waste water harmless; and a pigment used therefor.
In recent years, there have been a variety of needs for concrete constructions because of advancement of industrial structure and improvement in people's life in association with the development of industry and economy. In consequence, the concrete constructions have been demanded to satisfy conformity to natural environment or beauty.
In order to meet such a demand, there has been an increasing tendency that dry-as-dust concrete constructions are colored so as to be harmonized with nature or enhance the beauty or favorableness. For this reason, colored concrete constructions have been presently spotlighted.
Hitherto, the colored concrete constructions have been constructed using a colored concrete which is prepared by adding not more than 10% by weight of powdery or granular coloring pigment to a cement admixture composed usually of cement, coarse aggregate (gravel), fine aggregate (sand), water and various additives such as an admixing agent, and then kneading the resultant mixture. The colored concrete has been widely used in various constructions such as buildings, pavements of park promenades, roads, bulkheads for shore or bank protection, and the like, mainly for purposes of conformity to natural environment and beauty.
Meanwhile, in the production of the colored concrete, a residue of the colored concrete (which is composed substantially of mortar containing almost no coarse aggregate and the like) has attached to concrete production facilities such as a mixer or a belt conveyer, or to an inside of drum of a ready-mixed concrete conveying truck (agitator truck). Consequently, in the case where it is intended to produce other colored concrete or ordinary plain concrete in same production facilities or agitator trucks after the production of the colored concrete, it is necessary to recover the colored concrete residue attached thereto for its reuse, or wash out or remove the colored concrete residue therefrom.
As the method for recovering a concrete residue for its reuse, in Attachment 10 of JIS A 5308, there has been described a method of using mortar (concrete residue) which attaches to an inside of an agitator truck. In the method described in JIS A 5308, after concrete is completely unloaded, i.e., a whole amount of concrete is discharged from the agitator truck, a mortar residue which attaches to drum inner walls, mixing blades and the like of the agitator truck is slurried using a mortar stabilizer and then the resultant slurry is mixed with a fresh ordinary plain concrete.
However, in the case where the colored concrete residue, there arises a problem that when a fresh ordinary plain concrete (uncolored concrete) is loaded and mixed with the colored concrete residue, the resultant mixed concrete is wholly colored by pigments contained in the colored concrete residue, thereby making the resultant mixed concrete non-reusable.
On the other hand, in the case where the colored concrete residue attached is washed out, it has been demanded that colored concrete-containing waste water generated upon cleaning the production facilities for ready-mixed concrete, such as mixers, belt conveyers and the like, or an inside of drum of the agitator truck, should be purified and rendered harmless before being discharged from these production facilities or agitator trucks, or should be recovered for reuse thereof.
However, in the case of the colored concrete-containing waste water, since a supernatant liquid obtained by separating cement, fine aggregate and the like from the waste water is disadvantageously colored, it is impossible to reuse the supernatant liquid as a kneading water. Further, operations for purifying the supernatant liquid and making it harmless are complicated and it is extremely difficult to purify the supernatant liquid and make it harmless. Incidentally, the reason why it is difficult to purify the supernatant liquid is considered such that a coloring pigment contained in the colored concrete residue is readily dispersed in the supernatant liquid by a cement dispersant (admixing agent) usually mixed into a cement admixture of the colored concrete upon the production thereof. It is further considered that in the case where the coloring pigment is surface-treated in order to enhance its dispersibility in the cement admixture, the supernatant liquid is more readily colored.
In the field of mortars composed usually of cement, sand and water, which contain no coarse aggregate, colored mortars also have the same problems as described hereinbefore.
As described above, technical problems in the fields of colored concrete production are how to recover and reuse the colored concrete residue or colored mortar residue which attaches to production facilities thereof, or how to purify the colored concrete-containing waste water and render it harmless to discharge out of the production facilities or to obtain a colorless transparent supernatant liquid for its reuse as a kneading water. Nevertheless, no simple and effective methods therefor has been proposed.
As a result of the present inventor's earnest studies for solving the above-mentioned problems, it has been found that by adding to the colored concrete residue (i) a titanium oxide pigment, or (ii) a titanium oxide pigment and a pigment having a complementary color relative to the color of a coloring pigment contained in a colored concrete, the color of a hardened concrete containing the colored concrete residue can be similar to that of an ordinary plain concrete. The present invention has been attained based on the finding.